This invention relates generally to magnetic resonance imaging equipment, and more particularly to a method for reducing transient noise that interferes with the desired signal and may decrease the quality of the image that is produced.
Magnetic resonance imaging, or xe2x80x9cMRI,xe2x80x9d is an excellent medical diagnostic tool that has been around for several decades. The details of MRI are well-known and need not be repeated herein. In general, MRI involves placing a subject, such as a person, in a magnetic field of known strength. The hydrogen atoms in the subject, which are typically the atoms that are used for imaging in current MRI machines, will have a resonant frequency that is directly proportional to the applied magnetic field. By xe2x80x9cshapingxe2x80x9d the static magnetic field through the use of gradient coils, it is possible to produce a static magnetic field of known quantity at a single isolated region within the subject. This region is generally referred to as a voxel, and may be on the order of one cubic millimeter. By imaging thousands of these individual voxels, an overall image of the subject can be recreated.
The imaging of an individual voxel involves applying a radio frequency to the subject that corresponds to the resonant frequency of the voxel undergoing imaging. This resonant frequency is also known as the Larmor frequency. A certain number of hydrogen atoms in the voxel being imaged will absorb energy from the radio signal, which will cause them to switch spin states from a low energy state to a high energy state. After the radio signal is terminated, a certain number of hydrogen atoms in the high energy state will relax back to the low energy state, giving off a signal of known frequency during this relaxation process. By detecting this emitted signal, it is possible to determine the relative hydrogen content of the voxel being imaged. If the subject being imaged is a human, the different concentrations of hydrogen in the different human tissues will produce different signals for the voxels of different tissues. The different signals allow an image to be reconstructed such that it corresponds to the different tissues in the human body.
The signal emitted by the hydrogen atoms when relaxing from a high energy state to a low energy state is detected by a receiving antenna or coil that is positioned around the subject being imaged. In the case of MRI""s designed for imaging humans, the receiving antenna or coil is generally cylindrically shaped with the person positioned in the center of the cylinder. The MRI machine may contain a number of different coils of different size, location, and configuration in order to image different parts of the human body. In addition to the signals emitted by the relaxing hydrogen atoms, the detector coils or antennas will sense additional noise or interference signals. These noise or interference signals are desirably removed from the detected signal in order to produce a better image.
One prior art method for reducing the noise or interference in the receiving antennas is disclosed in U.S. Pat. No. 5,525,906 issued to Crawford et al., the disclosure of which is hereby incorporated herein by reference. In this method, which is depicted in block diagram in FIG. 3 herein, the signal from the receiving antenna is split into a detect path signal 1020 and a receive path signal 1022. The detect path 1020 passes through a band pass filter 1024 which removes broad band thermal noise from the detect path signal 1020. The detect path signal 1020 then passes through an amplifier 1026 before being input into a notch or band reject filter 1028. Notch filter 1028 is designed to reject all frequencies that occur within the desired signal frequency range, which has a known bandwidth. The output 1030 of filter 1028 will thus consist of unfiltered noise. The unfiltered noise 1030 is input into a comparator 1032 which compares this signal to a voltage threshold 1034. If the unfiltered noise signal 1030 exceeds the voltage threshold 1034, comparator 1032 outputs a signal at 1036 that causes switch SW1 to open, thereby blanking the output 1038. If the unfiltered noise signal 1030 does not exceed the voltage threshold 1034, the comparator outputs signal 1036, which leaves switch SW1 closed such that the receive signal 1022 is passed through to output 1038, after passing through delay filter 1040. The purpose of delay filter 1040 is to delay the signal on the receive path 1022 from reaching switch SW1 prior to comparator output signal 1036 reaching switch SW1. Such a system is described in more detail in the U.S. Pat. No. 5,525,906, particularly in reference to FIGS. 3 and 4 in the corresponding disclosure therein. While this prior art method has been successful in producing images of higher clarity, the need still exists for improved imaging techniques.
Accordingly, the present invention provides an improved method and apparatus for increasing the quality of MRI images. The present invention achieves this improved quality by providing an improved method for detecting transient noise that is generated in the MRI system.
According to one embodiment of the present invention, a method is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The method comprises detecting a parameter of the MRI signal that varies as the envelope of the MRI signal varies and filtering the MRI signal to thereby produce a filtered parameter signal. The filtered parameter signal is them compared to a reference signal. The MRI signal is determined to likely include interference if the filtered parameter signal exceeds the reference signal.
According to another aspect of the invention, a method is provided for detecting transient interference in an MRI signal that comprises detecting an envelope of the MRI signal and filtering out low frequency components of the MRI signal to thereby produce a filtered envelope signal. The filtered envelope signal is compared to a reference signal and it is determined that the MRI signal includes interference if the filtered envelope signal exceeds the reference signal.
According to still another aspect of the invention, an interference detection system is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The system includes a filter designed to remove low frequency components within the MRI signal, and an envelope detector that detects the envelope of the MRI signal. The filter and envelope detector produce in combination a filtered envelope signal. A comparator compares the filtered envelope signal to a reference signal and outputs an interference signal if the filtered envelope signal exceeds the reference signal.
According to yet another aspect of the invention, an interference detection system is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The system comprises a filter designed to remove low frequencies within the MRI signal and a detector that detects a parameter that varies as the envelope of the MRI signal varies. The filter and detector produce in combination a filtered parameter signal. A comparator compares the filtered parameter signal to a reference signal and outputs an interference signal if said filtered parameter exceeds the reference signal.
In still other aspects of the invention, the parameter detector and/or the envelope detector may comprises a detector log video amplifier. The system may include a blanking switch controlled in a manner to blank the MRI signal if the comparator outputs the interference signal. The system may further includes a retriggerable multivibrator that is activated by the interference signal.
The methods and systems of the present invention provide improved clarity in MRI images by more accurately discerning whether or not the signal in an MRI receiving coil is corrupted by transient noise. By more accurately determining whether transient interference is present, appropriate steps can be taken from preventing these transient interference signals from being used to produce image data. These and other advantages of the present invention will be apparent to one skilled in the art in light of the following specification when read in conjunction with the accompanying drawings.